Numerous methods are known in the prior art for monitoring analyte concentrations in body fluids. On the one hand there are systems in which blood is withdrawn by a catheter and conveyed to a measuring cell. The document WO 91/16416 which describes an instrument that can be carried on the arm that withdraws blood samples by means of a catheter implanted in a blood vessel is mentioned as a representative of such procedures. The sample liquid is conveyed through an essentially closed channel system to an enzyme electrode which is designed to carry out a multitude of measurements. The system described in this document and other systems based on electrochemical sensors that measure continuously, have the disadvantage that the sensors have a pronounced signal drift. This becomes particularly obvious from the document WO 91/16416 when the laborious calibration is taken into consideration. Another disadvantage of such sensor-based systems is that relatively large amounts of fluids are required. In the prior art sensors are known as systems that only require small amounts of liquid and thus this statement is initially surprising. However, when emphasising the positive features of sensor systems, one often does not take into account that fluid channels are necessary and a sensor surface of sufficient size has to be wetted.
Ultrafiltration devices are also known in the prior art of which the documents U.S. Pat. No. 4,832,034 and U.S. Pat. No. 4,777,953 are mentioned as examples. These systems also use electro-chemical sensors and thus also have the above-mentioned disadvantages. In addition there are disadvantages which are caused by the ultrafiltration membrane. It is critical to select a suitable membrane material which has the combined properties of an adequately high filtration effect and permeability and does not already become blocked after a short period.
Another procedure for monitoring analyte concentrations is known under the name microdialysis. Representative documents from this field are: U.S. Pat. No. 5,174,291, EP 0 401 179 and U.S. Pat. No. 4,265,249. Flow measuring cells with electrochemical sensors are used in the arrangements described in these documents. Although the ultrafiltration problems caused by membranes are less with microdialysis, microdialysis systems have the disadvantage that a perfusion liquid has to be pumped through a hollow catheter. The provision of solutions, the pumping process and the construction of the catheter are technical′ complications which increase the complexity.
The methods described above for monitoring analyte concentrations in body fluids are based on the premise that the monitoring requires a continuous or at least a more or less continuous measurement at relatively short time intervals. This explains the exclusive use of sensors that operate continuously in flow measuring cells.
Discontinuous concepts are also known in the field of analyte concentration monitoring. For example diabetics carry out several discrete measurements during a day in order to monitor their blood glucose level. For this purpose is it customary to firstly make an incision with a lancet and to apply the emerging blood to a disposable test element. This is analysed with a suitable device in order to determine the blood glucose concentration. Optical systems as well as systems that use electrochemical test elements are known in the prior art. Devices have also been known for some time in which the incision, sample collection and sample application can be carried out with a single disposable test element. Such systems for determining blood glucose in interstitial fluid are described for example in the documents U.S. Pat. No. 5,746,217, U.S. Pat. No. 5,823,973 and U.S. Pat. No. 5,820,570. The aforementioned devices have a thin cannula which is inserted into the dermis and collects interstitial fluid at this site. The cannula conveys the liquid onto a test element. A disadvantage of this system is that a cannula has to be inserted again for each individual measurement. In addition to the discomfort caused by the repeated piercing, the user has to carry out a number of operating steps such as inserting a disposable element into an apparatus, starting the lancing process, waiting until the result of the analysis is displayed and replacing the test element. Moreover the said devices have to be carried around by the user and he has to find a discreet place to carry out the measurement if he does not want to publicly exhibit his disease.
A system which also has the aforementioned disadvantages, but which uses a system comprising a catheter and an initially separate test element is described in U.S. Pat. No. 5,368,029. According to this document a catheter is firstly introduced into a blood vessel and one waits until a transparent chamber is filled with blood (flushing). Then a disposable test element is inserted into the chamber through a valve slit in order to bring the test element into contact with the blood. It is hardly conceivable that such a system could be used routinely by a diabetic since it is necessary to introduce a catheter into a blood vessel with a considerable risk of infection and injury. In addition a relatively large amount of sample is required. The description in the document shows that the system is designed to be used in emergency medicine. Another essential disadvantage of the concept is that the system does not enable monitoring of an analyte concentration but only allows a single measurement which reflects the momentary concentration level. The document contains no information or suggestions whatsoever on how to carry out repeated measurements by coupling new test elements. This is logical since the blood collected in the chamber is not exchanged and thus subsequent measurements with additional test elements would only yield the same measured value and not a measured value that would lead to a later concentration value.